Pressurized screens which include a cylindrical casing having a cylindrical screen plate mounted within it to define an annular space into which the material to be screened is received are well known in the art. In apparatus of this type rotating foils may be provided adjacent either the inner or outer face of the screen plate and flow may either be outwardly through the screen plate into the annular chamber formed by the screen plate outer face and the inner surface of the casing or inwardly in the opposite direction. Examples of apparatus of this latter type are shown in U.S. Pat. Nos. 3,174,622 of Mar. 23, 1965; 3,394,809 of July 30, 1968; 3,581,903 of June 1, 1971; 3,898,157 of Aug. 5, 1975; and 4,127,479 of Nov. 28, 1978.
One difficulty encountered with apparatus of the type described generally above is that where the flow is inwardly through the screen plate and the pulp is introduced through a wall of the casing and the rejects are removed through another opening there may be a short circuiting effect in that pulp may flow directly from the inlet to the reject outlet without first circling the screen plate. This will obviously result in substantial loss of good fibers.
Another disadvantage encountered in screens of this type is that as the paper making pulp is introduced into the chamber at a high velocity from the inlet it strikes the surface of the cylindrical screen plate, driving undesirable particles into the openings through the screen plate and either thereby passing undesirable particles through the screen plate by force or plugging some of the screen plate openings.
In some apparatus of this type the geometry of the chamber defined by the outer surface of the screen plate and the inner surface of the casing is such that eddies and general hydraulic inefficiency exists in the chamber about the screen. While U.S. Pat. Nos. 3,394,809 and 3,898,157 utilize spiral ramps to organize flow in the area about the screen plate, the resulting assembly is necessarily complex with attendant difficulties in manufacture and maintenance.